no case of humans in one piece improving their "durability "
you cant improve how hard your bones are......
Ofc you can, I doubt as Kid Sanji had harder durability then as he had in WCi.
He improve in phyiscal states, his body became more durable.
Best example Zoro, attacks who hurt him pre ts wouldn´t even tickel him after the ts, that is how superhumans get improve.
Sanji had so far no exekelton improve, the doc back then even said it:
He had a durability of a human.
His exekelton kicked out after he wear the raidsuit, he even said it:
All what Sanji gain, like speed,power, durability, he gain it as human without the exekelton improve a single time.
So all what we had pre Wano was Sanji who his own durabiility, not from the exekelton.
Like Sentinal said.
Human Sanji powers+modified powers>Siblings...
Sanji train himself as human, never had the attribute like his siblings.